Mid-latitude Electric Field Response during Isolated Substorms: Effects of SCW Location and Shielding

The substorm current wedge (SCW) characterizes the current system during substorms. The high-latitude electric field associated with the SCW penetrates toward mid- and low-latitudes. Various case studies have reported that the penetration electric field becomes stronger with the region-1 (R1) sense SCW, while the region-2 (R2) sense SCW shields the electric field. However, the statistical properties of the penetration electric field remain unclear. Moreover, the detailed relationship between the SCW's location and the electric field has been seldom examined. In this study, using substorms that occurred from 2010 to 2013, we evaluated the effects of shielding and statistically investigated its impact on the mid-latitude electric field using the following methods. We determined the temporal development of the SCW structure from the north-south and east-west components of the ground magnetic field and evaluated the effect of shielding, based on the magnitude of the R1/R2 SCW by AMPARE. We then analyzed the relation between these and the direction and magnitude of the electric field from Kyushu University’s FM-CW radar at midlatitude. The results show that, when the R1 SCW is more dominant, a westward electric field is observed in the center of the SCW and an eastward electric field is observed on the outside. The magnitude of those electric fields depends on the scale of the substorms. When the R2 SCW is comparable to the R1 SCW, these electric fields are shielded or overshielded, resulting in a smaller magnitude or opposite direction of the electric field. These findings suggest that the strength of the R2 SCW and the location of the SCW are the major contributors to the penetrating electric field.